Intermetallic Composite Formation and Fabrication from Nitride-Metal Reactions

ABSTRACT

In a method of making a molybdenum, molybdenum silicide and molybdenum silicon boride composite material, a boron nitride powder, a silicon nitride powder and a molybdenum powder are mixed to form a composite precursor. The composite precursor is sintered in an atmosphere consisting essentially of hydrogen and argon to form a sintered material. The sintered material is hot isostatic pressed to form the composite material into a final shape.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/947,503, filed Jul. 2, 2007, the entirety ofwhich is hereby incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with support from the U.S. government undergrant number NAVAIR N00421-041-0002, awarded by the Office of NavalResearch. The government may have certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite materials and, morespecifically, to a composite material including molybdenum, molybdenumsilicides and molybdenum silicon boride.

2. Description of the Prior Art

Nickel super alloy turbine blades in modern jet engines operate attemperatures approaching 1150° C. Using technologies such as thermalbarrier coatings and elaborate cooling schemes, such engine temperaturesmay be pushed as high as 1500° C. Dramatic increases in the performanceof such engines can be gained by operating at higher temperatures, butas more elaborate systems are employed to cool the airfoils, engineefficiency is reduced.

Refractory metals such as molybdenum have high melting points andexcellent high temperature mechanical properties; however, the oxidationresistance of these metals is typically poor. Although molybdenum (Mo)by itself can not be used in high-temperature oxidizing environments,compounds of molybdenum and silicon (Si) are known for their excellentoxidation resistance due to the formation of a protective silicateglass. Adding boron (B) improves oxidation resistance by decreasing theviscosity of the glass and promoting better surface coverage. A 1600° C.isothermal section of a Mo—Si—B ternary phase diagram 100 is shown inFIG. 1. The desired properties of turbine blades may be found in Mo—Si—Bcompositions in the molybdenum rich corner 102 of the phase diagram 100.This region includes three phases of matter: molybdenum solid solution(Mo_(ss)) and two intermetallic phases—Mo₃Si (referred to by those ofskill in the metallurgical and ceramic arts as “A15”) and Mo₅SiB₂(referred to as “T2”). These three phases have melting points above2000° C. and the phase field is stable down to room temperature, makingthese alloys excellent candidates for high temperature structural use.The fracture toughness of the intermetallic phases may be improved bythe presence of the more ductile molybdenum phase.

A variety of methods for producing Mo—Si—B alloys have emerged. Toachieve high strength and fracture toughness, the alloys must beprocessed in a manner that creates a continuous molybdenum matrix. Inaddition, a fine dispersion of the Mo—Si—B intermetallic phases isnecessary to maintain a continuous protective glass layer. Much of theresearch has focused on melt-based processing or consolidation ofpre-alloyed powders formed by inert gas atomization. Molybdenum has thehighest melting point of the three phases in the alloy, causing primarysolidification of the molybdenum solid solution. The resultingmicrostructures produced by these methods are coarse grained withisolated molybdenum regions.

Unfortunately, existing composites employing these phases have grainstructures that are too large for many practical applications. Unlikemany other structural alloys, Mo—Si—B alloys do not lend themselves tomicrostructural improvement by heat treating. Their compositionalhomogeneity over a wide temperature range eliminates the possibility formicrostructural reformation via phase transformations. Thus, this systemis similar to a ceramic, in that the initial synthesis methods largelydictate the final microstructure.

Powder metallurgy methods may provide an opportunity for microstructurecontrol. However, existing methods of creating Mo—Si—B compositematerials by powder processing routes have met with little success.Impurity levels are difficult to control because fine silicon and boronpowders are prone to oxidation during processing. Also, the segregationof carbon and oxygen at grain boundaries is known increase the ductileto brittle transition temperature (DBTT) of molybdenum. High residualoxygen levels of 3000 ppm for alloys produced by existing mechanicalalloying of elemental powders leads to a significant quantity of silicainclusions in the composite. A glass phase present in the bulkmicrostructure may also harm high temperature creep resistance.

Therefore, there is a need for a method of producing a composite thatincludes Molybdenum, A15 and T2 while minimizing surface oxidation.

Therefore, there is a need for a composite that includes Molybdenum, A15and T2 with small particle sizes.

Therefore, there is a need for highly dense composite structures thatinclude Molybdenum, A15 and T2.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present inventionwhich, in one aspect, is a method of making a molybdenum, molybdenumsilicide and molybdenum silicon boride composite material, in which aboron nitride powder, a silicon nitride powder and a molybdenum powderare mixed to form a composite precursor. The composite precursor issintered in an atmosphere consisting essentially of hydrogen and aninert gas to form a sintered material. The sintered material is hotisostatic pressed to form the composite material into a final shape.

In another aspect, the invention is a composite material having an outersurface and including a metallic phase continuous molybdenum matrix, amolybdenum silicide intermetallic phase, a molybdenum silicon borideintermetallic phase and a borosilicate glass layer. The metallic phasecontinuous molybdenum matrix has an average grain size of less than 4.0microns. The molybdenum silicide intermetallic phase has an averagegrain size of less than 2.5 microns and suspended in the metallic phasecontinuous molybdenum matrix. The molybdenum silicon borideintermetallic phase has an average grain size of less than 2.0 micronsand suspended in the metallic phase continuous molybdenum matrix. Theborosilicate glass layer covers at least a portion of the outer surface.

In yet another aspect, the invention is a mechanical structure that hasan outer surface. The mechanical structure includes a composite materialthat includes a molybdenum silicide intermetallic phase, a molybdenumsilicon boride intermetallic phase, and a metallic phase continuousmolybdenum solid solution matrix. The composite material has been coldisostatic pressed from Mo, Si₃N₄ and BN, into a form of the mechanicalstructure, sintered and hot isostatic pressed so that the compositematerial has a sintered density that is greater than 94% of theoreticaldensity. A borosilicate glass layer covers at least a portion of theouter surface of the structure.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIG. 1 is a portion of a ternary phase diagram for molybdenum, boron andsilicon at 1600° C.

FIG. 2 is a flow chart showing a method of making a composite materialstructure.

FIGS. 3A-3D are a series of schematic diagrams showing the making of acomposite material structure.

FIG. 4 is a graph relating green density of a shape to pressing pressureapplied to the shape prior to sintering.

FIG. 5 is a micrograph showing composite precursor powders prior tosintering.

FIG. 6 is a micrograph showing a composite material according to onerepresentative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. As used in the description herein and throughout the claims,the following terms take the meanings explicitly associated herein,unless the context clearly dictates otherwise: the meaning of “a,” “an,”and “the” includes plural reference, the meaning of “in” includes “in”and “on.”

As shown in FIG. 2, one representative method of making a molybdenum,molybdenum silicide and molybdenum silicon boride composite material,includes mixing 200 a sub-micron boron nitride (such as BN) powder, asub-micron silicon nitride (such as Si₃N₄) powder and a sub-micronmolybdenum powder to form a composite precursor. The composite precursormay be milled 202 prior to the sintering action to break up agglomeratesof the boron nitride powder, the silicon nitride powder and themolybdenum powder. The silicon nitride powder and the molybdenum powdermay be mixed with a liquid (such as acetone, or other organic liquid) toform a suspension. An organic dispersant and binder, such as a methylmethacrylate copolymer, may be added to the suspension. A lubricant,such as stearic acid, may also be added to the suspension. Thesuspension may be spray dried 204 to form a homogenous powder mixture.

The homogenous powder mixture is cold isostatically pressed 206 to forma green body. (It is understood that the term “green body” is a termcommonly used in the ceramic arts, referring to an un-fired ceramicstructure. This term does not connote any specific color.) A graph 400relating green density to pressing pressure, as shown in FIG. 4, showsthat in cold isostatic pressing the green body, the optimal pressingpressure is about 70 psi. Pressures greater than that do not improvegreen density. Returning to FIG. 2, the green body is sintered 208 in anatmosphere consisting essentially of hydrogen and argon (or other inertgas, such as helium) to form a sintered structure. The sinteredstructure is then hot isostatically pressed 210 to form the compositematerial into a final shape.

After the hot isostatic pressing, the final shape may be exposed to anatmosphere including oxygen at a temperature greater than 1000° C.,thereby forming 212 a borosilicate glass layer over at least a portionof an outer surface of the final shape. This borosilicate glass reducessurface oxidation of the shape.

The resulting composite material will have a metallic phase of acontinuous molybdenum matrix in which the average grain size is lessthan 4.0 microns, a molybdenum silicide intermetallic phase (such asA15) in which the average grain size is less than 2.5 microns and thatis dispersed in the continuous molybdenum matrix, and a molybdenumsilicon boride intermetallic phase (such as T2) in which the averagegrain size is less than 2.0 microns and that is dispersed in thecontinuous molybdenum matrix. In the composite, silicon is concentratedin a range of between 1 wt. % to 5 wt. % and boron is concentrated in arange of between ½ wt. % to 2 wt. %. This process also results in thefinal shape having a sintered density greater than 94% of theoreticaldensity and preferably greater than 99% of theoretical density.

Also, a thin borosilicate glass layer covers at least a portion (andpreferably all) of the outer surface. In an alternative embodiment,aluminum may be added to the composite, so that it includes up to 10 wt.% (up to 5 wt. % in most embodiments) of aluminum.

In one embodiment, as shown in FIGS. 3A-3D, a mechanical structure (suchas a gas turbine blade, or other gas turbine component, may be made bycold isostatically pressing the above-mentioned powders in a mold 300having the shape of the structure to form a green body 302. The greenbody 302 is fired in a furnace 310 (such as an alumina tube furnace) inan atmosphere 312 including hydrogen and argon. The hydrogen and argongasses are passed through a titanium gettering furnace 314 to removeoxygen and other impurities. A gas trap 316 is employed to prevent backpropagation of impurities. The fired body 322 is then hot isostaticallypressed in a heated mold 320. The heated mold 320 is removed to exposethe final structure 330.

A micrograph 500 of the spray dried powder prior to shape formation isshown in FIG. 5 and a micrograph 600 of a resulting composite materialafter sintering is shown in FIG. 6.

Employing the above-disclosed methods results in the synthesis andcontrol of the intermetallic phases as fine, uniform dispersions in acontinuous molybdenum matrix using powder metallurgical methods.Submicron molybdenum, Si₃N₄ and BN powders are reacted to form Mo—Si—Balloys. The covalent nitrides are stable in oxidizing environments up to150° C. to 200° C., allowing for fine particle processing without theformation of silicon and boron oxides. At high temperatures bothnitrides have high equilibrium nitrogen partial pressures and small freeenergies that promote formation of A15 and T2. The intermetallic phasesare formed by the following reactions:

3 Mo+1/3 Si₃N₄→Mo₃Si+2/3 N₂

5 Mo+1/3 Si₃N₄+2BN→Mo₅SiB₂+5/3 N₂

The resulting composites have low impurity levels and havemicrostructures with a fine dispersion of intermetallics in a continuousmolybdenum matrix. This processing route allows for microstructuralcontrol through adjustments in processing, raw materials and firingparameters. The method disclosed herein uses common powder processingtechniques which are standard industry practice, making the processrelatively inexpensive and viable for scale up.

Reactant powders for Mo—Si—B alloys used in one experimental embodimentare listed in the following table:

Surface Raw Area Oxygen Material Grade Purity Particle Size (m²/g)Content Mo Ultra- 99.95% 100-500 nm 3.3  0.8 wt % (Climax) fine Si₃N₄SN-E03 99% ~0.5 μm (APS) 4 0.82 wt % (UBE) BN B-1084 99.5% 0.73 μm (APS)6.7 N/A (Cerac)Powder selection criteria were based on high purities and low oxygencontents, as well as being commercially available in large quantities.Submicron powders were chosen to lower sintering temperatures and tomaintain a fine grain sizes after firing.

A homogenous dispersion of the starting powders helped to achieve a finedispersion of the intermetallics phases in the final microstructure. Thepowders were mixed in acetone with 3 wt. % of low molecular weightmethyl methacrylate copolymer, Elvacite 2008 (available from LuciteInternational), which was added as a dispersant and binder. The Elvacite2008 resin burns out cleanly during firing by breaking down to themonomer and evaporating away, leading to low residual carbon levels.Stearic acid at 0.3 wt. % was added as a powder lubricant to reducedensity gradients in the pressed compacts. The mixtures were milled withAl₂O₃ media for 30 minutes on a commercial paint shaker to break upagglomerates and improve dispersion. The slurries were then spray driedin a small laboratory spray dryer (a BÜCHI Model 190) to maintain thefine dispersion of the starting powders. The spray dried powders werescreened to separate the spherical granules, which give a more uniformfill and reduce density gradients in dry pressing operations. Powdercompacts were uniaxially pressed to 480 MPa in a ½″ die using anautomated hydraulic press to ensure constant conditions for the loadingrate and hold time.

The resulting powder compacts were fired in a sealed atmosphere 1600° C.tube furnace. The firing profile was computer controlled and monitoredby a thermocouple placed directly above the center of the tube. Sampleswere heated at 3° C./min with a six hour hold at temperature. Oxygenconcentrations in the samples were minimized by firing in a reducingatmosphere of Ar-10% H₂. Hydrogen reduces molybdenum oxides, leading tolow residual oxygen levels. Ultra-high purity grade gases (availablefrom Airgas Products) had measured oxygen levels of 45 ppm and the inletgas stream was purified using a titanium gettering furnace which reducedthe measured oxygen content in the furnace outlet to value of 0.0 ppm.

Impurity levels of oxygen, carbon and nitrogen were measured for a Mo-3Si-1B wt. % sample fired at 1600° C. for six hours. The measured oxygenvalues of 300-400 ppm were lower than reported previously for Mo—Si—Balloys produced using powder metallurgy. Low oxygen levels are importantfor limiting silica inclusions in the microstructure.

Pellets with varying Mo contents were fired between 1200° C. and 1600°C. with six hour holds at each set point. The densities of the firedsamples were measured using the Archimedes method. The higher molybdenumcontent samples initially densified at a more rapid rate. The differencein sintering behavior may be due to the higher volume fraction of themolybdenum phase. After reaction, further densification of the alloy maybe inhibited by the presence of the intermetallic phases. Ultimately,the compositions achieved relative densities of 95% of theoretical. Atthe maximum furnace temperature of 1600° C., densities have not leveledoff and increasing the sintering temperature would result in higherdensities. However, this would also lead to increased grain growth.

To achieve densities approaching theoretical while maintaining a finegrain size, hot-isostatic pressing (HIP) was used after sintering. Forsamples above about 94% theoretical density the discontinuous porosityallows for HIPing without encapsulation. Mo-3Si-1B wt. % samples werepre-fired at 1600° C. and then HIPed at 1400° C. and 207 MPa for sixhours (using a press available from American Isostatic Presses,Columbus, Ohio). The average density of the samples increased from 94.1to 99.4% of theoretical with no significant grain growth.

In reaction studies, Molybdenum, Si₃N₄ and BN powders were combined toyield stoichiometric mixtures of the A15 and T2 phases. Samples wereheated in Ar-5% H₂ at 3° C./min to match the firing conditions used inthe furnace. Weight loss occurred up to 885° C. due to evaporation ofmolybdenum trioxide or by reduction of oxide. The onset of the reactionswith the nitrides is evidenced by a second stage of weight loss due tothe evolution of nitrogen from the mixtures. The A15 precursor mixturebegan reacting at 1193° C. and the T2 precursor mixture began reactingat 1140° C.

The formation of Mo—Si—B alloys by reaction of molybdenum, Si₃N₄ and BNpowders has been demonstrated to produce Mo-T2-A15 composites withpressureless sintered densities greater than 94% of theoretical density.Use of the nitrides allows for fine particle processing both in formingsteps with organic additives and sintering without oxidation. Overallimpurity contents were maintained at low levels. The methods describedprovide a means for creating these materials in a much less complex andexpensive manner than has been previously demonstrated. Theintermetallic phases improve high-temperature creep resistance andprovide oxidation resistance by forming a protective borosilicate glasssurface layer.

It should be noted that by: (1) controlling particle size and shape ofnew material; (2) controlling heat treatment temperature and time; and(3) employing hot isostatic conditions, reduction of Mo, A15 and T2grain sizes to 0.1 microns may be achieved while achieving densities ofgreater than 99% of theoretical.

The above described embodiments, while including the preferredembodiment and the best mode of the invention known to the inventor atthe time of filing, are given as illustrative examples only. It will bereadily appreciated that many deviations may be made from the specificembodiments disclosed in this specification without departing from thespirit and scope of the invention. Accordingly, the scope of theinvention is to be determined by the claims below rather than beinglimited to the specifically described embodiments above.

1. A method of making a molybdenum, molybdenum silicide and molybdenumsilicon boride composite material, comprising the actions of: a. mixinga boron nitride powder, a silicon nitride powder and a molybdenum powderto form a composite precursor; b. cold isostatic pressing the compositeprecursor thereby forming a cold isostatic pressed precursor; c.sintering the cold isostatic pressed precursor in an atmosphereconsisting essentially of hydrogen and an inert gas to form a sinteredmaterial; and d. hot isostatic pressing the sintered material to formthe composite material into a final shape.
 2. (canceled)
 3. The methodof claim 1, further comprising the action of exposing the final shape toan atmosphere including oxygen at a temperature greater than 1000° C. soas to form a borosilicate glass layer over at least a portion of anouter surface of the final shape.
 4. The method of claim 1, wherein eachof the boron nitride powder, the silicon nitride powder and themolybdenum powder consist essentially of granules that are sub-micron insize.
 5. The method of claim 1, further comprising the action of millingthe composite precursor prior to the sintering action to break upagglomerates of the boron nitride powder, the silicon nitride powder andthe molybdenum powder, thereby making a homogeneous dispersion of theboron nitride powder, the silicon nitride powder and the molybdenumpowder.
 6. The method of claim 5, wherein the action of making ahomogeneous dispersion comprises the actions of: a. mixing the boronnitride powder, the silicon nitride powder and the molybdenum powderwith a liquid to form a suspension; and b. spray drying the suspensionto form a homogenous powder mixture.
 7. The method of claim 6, whereinthe liquid comprises an organic liquid.
 8. The method of claim 7,wherein the organic liquid comprises acetone.
 9. The method of claim 6,further comprising the action of mixing an organic dispersant and binderwith the suspension prior to the spray drying action.
 10. The method ofclaim 9, wherein the organic dispersant and binder comprises a methylmethacrylate copolymer.
 11. The method of claim 6, further comprisingthe action of mixing a lubricant with the suspension prior to the spraydrying action.
 12. The method of claim 11, wherein the lubricantcomprises stearic acid.
 13. The method of claim 1, wherein the boronnitride powder consists essentially of BN.
 14. The method of claim 1,wherein the silicon nitride powder consists essentially of Si₃N₄.
 15. Acomposite material having an outer surface, comprising: a. a metallicphase continuous molybdenum matrix having an average grain size of lessthan 4.0 microns; b. a molybdenum silicide intermetallic phase having anaverage grain size of less than 2.5 microns and suspended in themetallic phase continuous molybdenum matrix; c. a molybdenum siliconboride intermetallic phase having an average grain size of less than 2.0microns and suspended in the metallic phase continuous molybdenummatrix; and d. a borosilicate glass layer covering at least a portion ofthe outer surface.
 16. The composite material of claim 15, wherein themolybdenum silicide intermetallic phase comprises A15.
 17. The compositematerial of claim 15, wherein the molybdenum silicon borideintermetallic phase comprises T2.
 18. The composite material of claim15, wherein silicon is concentrated in a range of between 1 wt. % to 5wt. %.
 19. The composite material of claim 15, wherein boron isconcentrated in a range of between 1/2 wt. % to 2 wt. %.
 20. Thecomposite material of claim 15, wherein the metallic phase continuousmolybdenum matrix comprises molybdenum and up to 10% by weight ofaluminum.
 21. The composite material of claim 15, having a sintereddensity greater than 94% of theoretical density.
 22. A mechanicalstructure having an outer surface, comprising: a. a composite materialthat includes a molybdenum silicide intermetallic phase, a molybdenumsilicon boride intermetallic phase, and a metallic phase continuousmolybdenum solid solution matrix, the composite material having beencold isostatic pressed into a form of the mechanical structure, sinteredand hot isostatic pressed so that the composite material has a sintereddensity that is greater than 94% of theoretical density; and b. aborosilicate glass layer covering at least a portion of the outersurface of the structure.
 23. The mechanical structure of claim 22,wherein the molybdenum silicide intermetallic phase comprises A15. 24.The mechanical structure of claim 22, wherein the molybdenum siliconboride intermetallic phase comprises T2.
 25. The mechanical structure ofclaim 22, wherein silicon is concentrated in a range of between 1 wt. %to 5 wt. %.
 26. The mechanical structure of claim 22, wherein boron isconcentrated in a range of between 1/2 wt. % to 2 wt. %.
 27. Themechanical structure of claim 22, configured as part of a gas turbineengine.